Such rotation transmission devices include a two-way clutch type rotation transmission device comprising a roller clutch including an inner member, an outer ring and rollers disposed between the inner member and the outer ring, and an electromagnetic clutch. Torque is selectively transmitted between the inner member and the outer ring by selectively energizing the electromagnetic clutch and thus engaging the roller clutch. It is desired that the electromagnetic clutch consume less energy and produce less heat. It is also desirable that the electromagnetic clutch function as a heater in a low temperature environment. The invention disclosed in Patent document 1, which is titled “Method for controlling a rotation transmission device” proposes a rotation transmission device which satisfies these requirements.
Patent document 1 proposes to save energy and reduce heat build-up by intermittently applying current to the electromagnetic coil to keep the two-way clutch engaged, thereby keeping the vehicle in its 4WD mode. Current applied to the electromagnetic coil is controlled by pulse width modulation (PWM). The two-way clutch of Patent publication 1 is mounted in a transfer case of a 4WD vehicle of the front-engine, rear-drive layout. But such two-way clutches can be used for various other devices.
Patent document 2 discloses a method for controlling a 4WD vehicle using a rotation transmission device similar to the one disclosed in Patent publication 1. In this control method, if 2WD mode is selected while the ambient temperature is low and thus the oil temperature in the transfer case, as detected by a temperature sensor, is lower than a predetermined value, hub clutches or a front axle engaging means is locked to prevent vibrations of the vehicle due to repeated locking and unlocking of the two-way roller clutch.
In the control method based on pulse width modulation of Patent document 1, (a) when the two-way roller clutch is locked by energizing the electromagnetic coil, a relatively large current is applied to the electromagnetic coil to lock the roller clutch as quickly as possible, and (b) once the roller clutch locks, a relatively small current, i.e. current sufficient to produce a magnetic attraction force barely larger than the force of the switch spring from the electromagnetic clutch is applied to the electromagnetic coil. The current in either instance is determined according to the type of the clutch without taking into consideration the revolving speed of the rotary shaft when the roller clutch locks.
The inventors conducted studies on current values to be applied to the electromagnetic clutch and discovered that it is possible to further reduce power consumption if the current to be applied to the electromagnetic coil to lock the roller clutch is determined taking into consideration the revolving speed of the rotary shaft. None of prior proposals in the art of rotation transmission devices takes into consideration the influence of the revolving speed of the rotary shaft in determining the current to be applied to the electromagnetic coil to lock the roller clutch.
Patent document 2 discloses that the electromagnetic coil can be used as a heater by applying current to the electromagnetic coil while the ambient temperature is low. Irrespectively of whether the vehicle is traveling or at a stop, simply by energizing the electromagnetic coil, the coil can be used as a heater to heat lubricating oil and thus to instantly reduce its viscous resistance. But when the electromagnetic coil is energized, the coil also produces magnetic attraction force as an electromagnet. Thus, if a large current is applied to produce high calorie heat, the armature may be attracted to the rotor, causing untimely locking of the roller clutch. It is therefore desired to determine the current to be applied to the electromagnetic coil to an optimum value and to provide an electromagnet which can be used as a heater in a low-temperature environment without the possibility of untimely and undesired locking of the roller clutch.
In the above-described conventional rotation transmission device, because the two-way clutch and the electromagnetic clutch are mounted in an output member in a row, the output member has a large axial length. Further, because a nonmagnetic rotor guide is fitted in the output member so as not to be rotatable relative to the output member, and the rotor is fitted in the rotor guide to prevent leakage of the magnetic flux that flows through the rotor, the output member has a large diameter too. The entire rotation transmission device is thus heavyweight.
In order to solve these problems, Patent document 3 proposes to divide the output member into an outer ring and an outer member made of a nonmagnetic material such as synthetic resin and rotationally fixed to the outer ring, provide the two-way clutch between the outer ring and the input member and support the rotor of the electromagnetic clutch on the outer member. By dividing the output member into the outer ring and the nonmagnetic outer member, it is possible to reduce the weight of the rotation transmission device.
The rotation transmission device disclosed in Patent document 3 uses a coupling means for coupling the outer ring to the outer member, such coupling means comprising a radial pin through which the outer ring and the outer member are coupled together so as to be nonrotatable and axially immovable relative to each other, or comprising flat surfaces of the outer ring and the outer member that abut each other, thereby preventing relative rotation between the outer ring and the outer member, and a snap ring engaged in a ring groove formed in the outer periphery of the outer ring to prevent axial movement of the outer member relative to the outer ring. But such a pin or snap ring may separate under centrifugal force when the outer ring rotates at high speed, and thus cannot couple the outer ring and outer member together with sufficiently high reliability. It may be conceivable to replace the pin with a screw or to caulk the edge of the open end of the hole into which the pin is inserted. But either solution is expensive.
Patent document 1: JP patent publication 11-159545A
Patent document 2: JP patent publication 11-157355A
Patent document 3: JP patent publication 2001-311438A